While it is known that glucagon induces a biphasic release of insulin when infused into a normal animal, it is not known whether the resultant pattern of insulin secretion has important metabolic consequences. To ascertain whether such is the case, glucagon was elevated fourfold in the presence of first-, second-, or combined first- and second-phase insulin release to determine ability of the latter hormone to antagonize the effect of glucagon on glucose turnover in the conscious dog. To separate the effects of the different phases of insulin release the “pancreatic clamp” technique, in which somatostatin is given to inhibit the endocrine pancreas and replacement amounts of insulin and glucagon are given intraportally, was used. In this way, a rise in glucagon (∼220 pg/ml) was brought about in the presence of simulated first-phase (peak IRI 25 μU/ml at 5 min; basal by 30 min), secondphase (peak IRI 19 μU/ml at 30 min and sustained elevation thereafter), or first- plus second-phase (peak IRI 33 μU/ml at 5 min; 17 μU/ml at 30 min and sustained elevation thereafter) insulin release.
Optimal glycemic control required both first- and second-phase insulin release. A selective deficiency of first-phase release resulted in a transient (2 h) worsening of the glucagon-induced hyperglycemia (twofold the normal increment). This defect was attributable to a larger initial rise in glucose production (3.6 ± 0.6 mg/kg · min) than that observed when both phases of insulin release were present (0.9 ± 0.4 mg/kg · min). First-phase insulin release had no significant effect on glucose clearance. A selective deficiency of second-phase release resulted in marked (sixfold) and prolonged worsening of the glucagon-induced hyperglycemia. In this case, however, the hyperglycemia was primarily the result of a defect in glucose clearance. Glucose clearance fell by 29 ± 7% instead of rising by 30 ± 4% as it did when both firstand second-phase release were present. Glucose production was mildly elevated between 15 and 75 min when second-phase insulin release was deficient relative to that apparent when both the first- and secondphases of release were present; this also contributed to the abnormal hyperglycemia.
We conclude that both phases of insulin release are vital to full counterregulation of the action of glucagon on glucose metabolism. First-phase insulin release is important to counter the quick effect of glucagon on glucose production, while second-phase insulin release is important to sustain that inhibition and to augment glucose utilization. Absence of first-phase release results in a transient (2 h) and moderate (20–30 mg/dl) glycemic defect while an absence of second-phase release results in a prolonged and dramatic (70–100 mg/dl) defect.